When a pattern of a photoresist is formed by photolithography, if interference of light reflected on the boundary between a photoresist layer and a substrate and on the surface of the photoresist layer occurs at the time of exposure, the dimensional accuracy of the pattern will be decreased. For the purpose of reducing the reflected light, a method of forming an antireflection film on the surface of the photoresist layer has been studied. It is considered favorable that an antireflection film and a coating composition to be used for forming it have the following performance in general.
(1) It is found from a known theory regarding antireflection, the highest antireflection effect will be obtained when the refractive index of an antireflection film is equal to the square root of the refractive index of a photoresist. Since the refractive index of a common photoresist is from 1.6 to 1.8, the refractive index of an antireflection film is preferably at most 1.4.
(2) A coating composition is preferably in the form of an aqueous solution from such a viewpoint that no organic solvent has to be used in a clean room, whereby the safety can be secured and the environmental pollution can be avoided. Further, in order that it is practically used as a product, preferably it can be in the form of an aqueous solution over at least several months, i.e. it has long-term stability.
However, a conventional coating composition (see JP-A-11-124531 for example) contains methanol in addition to water as a medium in some cases, and such is not necessarily sufficiently satisfactory in view of the use in a clean room. Further, in the case of a conventional coating composition using a polymer having carboxyl groups, the polymer is neutralized by forming it into a form of an ammonium salt by using e.g. ammonia or tetramethylammonium hydroxide so as to impart water solubility to the polymer. Such a neutralized carboxyl group-containing polymer may have sufficient water solubility immediately after the production, but may aggregate with time, and such a polymer can hardly be used practically.